5,691 research outputs found
Beyond Gravitoelectromagnetism: Critical Speed in Gravitational Motion
A null ray approaching a distant astronomical source appears to slow down,
while a massive particle speeds up in accordance with Newtonian gravitation.
The integration of these apparently incompatible aspects of motion in general
relativity is due to the existence of a critical speed. Dynamics of particles
moving faster than the critical speed could then be contrary to Newtonian
expectations. Working within the framework of gravitoelectromagnetism, the
implications of the existence of a critical speed are explored. The results are
expected to be significant for high energy astrophysics.Comment: 13 pages, to appear in the Special December 2005 Issue of Int. J.
Mod. Phys.
Ultra--cold gases and the detection of the Earth's rotation: Bogoliubov space and gravitomagnetism
The present work analyzes the consequences of the gravitomagnetic effect of
the Earth upon a bosonic gas in which the corresponding atoms have a
non--vanishing orbital angular momentum. Concerning the ground state of the
Bogoliubov space of this system we deduce the consequences, on the pressure and
on the speed of sound, of the gravitomagnetic effect. We prove that the effect
on a single atom is very small, but we also show that for some thermodynamical
properties the consequences scale as a non--trivial function of the number of
particles.Comment: 4 page
HdC and EHe stars through the prism of Gaia DR3: 3D distribution and Gaia's chromatic PSF effects
Upon its release the Gaia DR3 catalogue has led to tremendous progress in
multiple fields of astronomy by providing the complete astrometric solution for
nearly 1.5 billion sources. We analysed the photometric and astrometric results
for Hydrogen-deficient Carbon (HdC), Extreme Helium (EHe), and DYPer type stars
to identify any potential biases and thus to select stars suitable for
kinematic and spatial distribution studies. We investigated the impact of
photometric declines in R Coronae Borealis (RCB) stars on Gaia astrometry using
information from the Gaia IPD process cross-matched with light curves. We have
reached the conclusion that the astrometric fits for numerous RCB stars are not
valid due to the Gaia PSF chromaticity effect in both shape and centroid. The
astrometric results of all stars with a significant time-dependent colour
variation should be similarly affected. RCB stars might thus be promising
sources to correct this effect in future Gaia releases. Furthermore, after
validating the Gaia astrometric results for 92 stars, we observed that the
majority of HdC and EHe stars are distributed across the three old stellar
structures, the thick disk, the bulge and the halo. However, we have also
uncovered evidence indicating that some of them exhibit orbits characteristic
of the thin disk. This is also particularly true for all DYPer type stars
studied. Finally, we have produced a list of star memberships for each Galactic
substructure, and provided a list of heliocentric radial velocities and
associated errors for targets not observed by Gaia DR3. We are beginning to
observe a relationship between kinematics, stellar population, and metallicity
in RCB and EHe stars. That relation can be explained, within the double
degenerate scenario, by the large range in the delay time distribution expected
from population synthesis simulations, particularly through the HybCO merger
channel.Comment: 22 pages, 20 figures, accepted for publication in A&A v2: RV section
from accompanying paper ArXiv2309.10139 moved into that one +New section
added on the comparison between our classification of HdC and EHe stars
within a Galactic substructure and the one published lately by Philip Monai
et al. (2023), and between both results on dynamical orbital parameters as
determined with Galpy simulation
HdC and EHe stars through the prism of Gaia DR3: Evolution of RV amplitude and dust formation rate with effective temperature
The Gaia DR3 release includes heliocentric radial velocity measurements and
velocity variability indices for tens of millions of stars observed over 34
months.In this study, we utilise these indices to investigate the intrinsic
radial velocity variations of Hydrogen-deficient Carbon (HdC) stars and Extreme
Helium (EHe) stars across their large ranges of temperature and brightness.
Taking advantage of the newly defined HdC temperature classes, we examine the
evolution of the total velocity amplitude with effective temperature.
Additionally, we analyse the variation in the dust production rate of R Coronae
Borealis (RCB) stars with temperature using two different proxies for the
photometric state of RCB stars: one from Gaia and another from the 2MASS
survey. Our observations revealed a trend in the evolution of the maximum
radial velocity amplitude across each HdC temperature class. Similarly, we also
observed a correlation between stellar temperature and the dust production
rate. Interestingly, we possibly observed for the first time some variations of
the intrinsic radial velocity amplitude and the dust production rate with HdC
temperature class. If confirmed, these variations would indicate that the
helium shell-burning giant stage starts with strong atmospheric motions that
decrease in strength, up to 6000 K, before picking up again as the HdC
star atmosphere shrinks further in size and reaches warmer temperatures.
Moreover, the dust formation rate appears to be much higher in colder RCB stars
compared to warmer ones.Comment: 7 pages, 4 figures, accepted in A&A V2: version accepted, radial
velocity section of V1 moved into the accompanying paper ArXiv2309.1014
Gravitomagnetism in teleparallel gravity
The assumption that matter charges and currents could generate fields, which
are called, by analogy with electromagnetism, gravitoeletric and
gravitomagnetic fields, dates from the origins of General Relativity (GR). On
the other hand, the Teleparallel Equivalent of GR (TEGR), as a gauge theory,
seems to be the ideal scenario to define these fields, based on the gauge field
strength components. The purpose of the present work is to investigate the
nature of the gravitational electric and magnetic fields in the context of the
TEGR, where the tetrad formalism behind it seems to be more appropriated to
deal with phenomena related to observers.
As our main results, we have obtained, for the first time, the exact
expressions for the gravito-electromagnetic fields for the Schwarzschild
solution that in the linear approximation become the usual expected ones. To
improve our understanding about these fields, we have also studied the geometry
produced by a spherical rotating shell in slow motion and weak field regime.
Again, the expressions obtained are in complete agreement with those of
electromagnetism.Comment: 25 pages. Submitted to International Journal of Modern Physics D.
Version 2: some new discussions, references adde
The DICE calibration project: design, characterization, and first results
We describe the design, operation, and first results of a photometric
calibration project, called DICE (Direct Illumination Calibration Experiment),
aiming at achieving precise instrumental calibration of optical telescopes. The
heart of DICE is an illumination device composed of 24 narrow-spectrum,
high-intensity, light-emitting diodes (LED) chosen to cover the
ultraviolet-to-near-infrared spectral range. It implements a point-like source
placed at a finite distance from the telescope entrance pupil, yielding a flat
field illumination that covers the entire field of view of the imager. The
purpose of this system is to perform a lightweight routine monitoring of the
imager passbands with a precision better than 5 per-mil on the relative
passband normalisations and about 3{\AA} on the filter cutoff positions. The
light source is calibrated on a spectrophotometric bench. As our fundamental
metrology standard, we use a photodiode calibrated at NIST. The radiant
intensity of each beam is mapped, and spectra are measured for each LED. All
measurements are conducted at temperatures ranging from 0{\deg}C to 25{\deg}C
in order to study the temperature dependence of the system. The photometric and
spectroscopic measurements are combined into a model that predicts the spectral
intensity of the source as a function of temperature. We find that the
calibration beams are stable at the level -- after taking the slight
temperature dependence of the LED emission properties into account. We show
that the spectral intensity of the source can be characterised with a precision
of 3{\AA} in wavelength. In flux, we reach an accuracy of about 0.2-0.5%
depending on how we understand the off-diagonal terms of the error budget
affecting the calibration of the NIST photodiode. With a routine 60-mn
calibration program, the apparatus is able to constrain the passbands at the
targeted precision levels.Comment: 25 pages, 27 figures, accepted for publication in A&
Observation of the Decay B^-âD_s^((*)+)K^-â^-ÎœÌ _â
We report the observation of the decay B^- â D_s^((*)+)K^-â^-ÎœÌ
_â based on 342ââfb^(-1) of data collected at the ΄(4S) resonance with the BABAR detector at the PEP-II e^+e^- storage rings at SLAC. A simultaneous fit to three D_s^+ decay chains is performed to extract the signal yield from measurements of the squared missing mass in the B meson decay. We observe the decay B^- â D_s^((*)+)K^-â^-ÎœÌ
_â with a significance greater than 5 standard deviations (including systematic uncertainties) and measure its branching fraction to be B(B^- â D_s^((*)+)K^-â^-ÎœÌ
_â)=[6.13_(-1.03)^(+1.04)(stat)±0.43(syst)±0.51(B(D_s))]Ă10^(-4), where the last error reflects the limited knowledge of the D_s branching fractions
New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars
R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich
supergiant stars that are increasingly thought to result from the merger of two
white dwarfs, called the Double degenerate scenario. This scenario is also
studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions.
Therefore a better understanding of RCBs composition would help to constrain
simulations of such events. We searched for and studied RCB stars in the EROS
Magellanic Clouds database. We also extended our research to DY Per type stars
(DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous
than their hotter counterparts. The light curves of ~70 millions stars have
been analysed to search for the main signature of RCBs and DYPers: a large drop
in luminosity. Follow-up optical spectroscopy was used to confirm each
photometric candidate found. We have discovered and confirmed 6 new Magellanic
Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14
DYPers. We estimated a range of Magellanic RCB shell temperatures between 360
and 600 K. We confirm the wide range of absolute luminosity known for RCB
stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are
ideal to search for RCB stars, since they have thinner and cooler circumstellar
shells than classical post-AGB stars. In addition, by increasing the number of
known DYPers by ~400%, we have been able to shed light on the similarities in
the spectral energy distribution between DYPers and ordinary carbon stars. We
also observed that DYPer circumstellar shells are fainter and hotter than those
of RCBs. This suggests that DYPers may simply be ordinary carbon stars with
ejection events, but more abundance analysis is necessary to give a status on a
possible evolutionnary connexion between RCBs and DYPers.Comment: 22 pages, 38 figures, Accepted for publication in A&
ExELS: an exoplanet legacy science proposal for the ESA Euclid mission. II. Hot exoplanets and sub-stellar systems
The Exoplanet Euclid Legacy Survey (ExELS) proposes to determine the
frequency of cold exoplanets down to Earth mass from host separations of ~1 AU
out to the free-floating regime by detecting microlensing events in Galactic
Bulge. We show that ExELS can also detect large numbers of hot, transiting
exoplanets in the same population. The combined microlensing+transit survey
would allow the first self-consistent estimate of the relative frequencies of
hot and cold sub-stellar companions, reducing biases in comparing "near-field"
radial velocity and transiting exoplanets with "far-field" microlensing
exoplanets. The age of the Bulge and its spread in metallicity further allows
ExELS to better constrain both the variation of companion frequency with
metallicity and statistically explore the strength of star-planet tides.
We conservatively estimate that ExELS will detect ~4100 sub-stellar objects,
with sensitivity typically reaching down to Neptune-mass planets. Of these,
~600 will be detectable in both Euclid's VIS (optical) channel and NISP H-band
imager, with ~90% of detections being hot Jupiters. Likely scenarios predict a
range of 2900-7000 for VIS and 400-1600 for H-band. Twice as many can be
expected in VIS if the cadence can be increased to match the 20-minute H-band
cadence. The separation of planets from brown dwarfs via Doppler boosting or
ellipsoidal variability will be possible in a handful of cases. Radial velocity
confirmation should be possible in some cases, using 30-metre-class telescopes.
We expect secondary eclipses, and reflection and emission from planets to be
detectable in up to ~100 systems in both VIS and NISP-H. Transits of ~500
planetary-radius companions will be characterised with two-colour photometry
and ~40 with four-colour photometry (VIS,YJH), and the albedo of (and emission
from) a large sample of hot Jupiters in the H-band can be explored
statistically.Comment: 18 pages, 16 figures, accepted MNRA
ExELS: an exoplanet legacy science proposal for the ESA Euclid mission. II. Hot exoplanets and sub-stellar systems
The Exoplanet Euclid Legacy Survey (ExELS) proposes to determine the
frequency of cold exoplanets down to Earth mass from host separations of ~1 AU
out to the free-floating regime by detecting microlensing events in Galactic
Bulge. We show that ExELS can also detect large numbers of hot, transiting
exoplanets in the same population. The combined microlensing+transit survey
would allow the first self-consistent estimate of the relative frequencies of
hot and cold sub-stellar companions, reducing biases in comparing "near-field"
radial velocity and transiting exoplanets with "far-field" microlensing
exoplanets. The age of the Bulge and its spread in metallicity further allows
ExELS to better constrain both the variation of companion frequency with
metallicity and statistically explore the strength of star-planet tides.
We conservatively estimate that ExELS will detect ~4100 sub-stellar objects,
with sensitivity typically reaching down to Neptune-mass planets. Of these,
~600 will be detectable in both Euclid's VIS (optical) channel and NISP H-band
imager, with ~90% of detections being hot Jupiters. Likely scenarios predict a
range of 2900-7000 for VIS and 400-1600 for H-band. Twice as many can be
expected in VIS if the cadence can be increased to match the 20-minute H-band
cadence. The separation of planets from brown dwarfs via Doppler boosting or
ellipsoidal variability will be possible in a handful of cases. Radial velocity
confirmation should be possible in some cases, using 30-metre-class telescopes.
We expect secondary eclipses, and reflection and emission from planets to be
detectable in up to ~100 systems in both VIS and NISP-H. Transits of ~500
planetary-radius companions will be characterised with two-colour photometry
and ~40 with four-colour photometry (VIS,YJH), and the albedo of (and emission
from) a large sample of hot Jupiters in the H-band can be explored
statistically.Comment: 18 pages, 16 figures, accepted MNRA
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